If you do decide to come to Belize, be sure to share your comparisons between Ambergris Caye and the Caymans. The Caymans have great shore snorkeling and some beautiful beaches, but I suspect you'll find San Pedro MUCH cheaper and much less developed and commercialized.
Of course you can get vegetarian food in San Pedro. Many restaurants have vegetarian options, plus there's plenty of pizza, pasta, fruit and of course beans and rice (though some may be cooked with lard or meat seasonings.)
--Lan Sluder